1. Field of the Invention
The present invention relates to a process for preparing polyarylene sulfides and, more particularly, to a process for preparing polyarylene sulfides having a high molecular weight and high stability, in which a surplus of water can readily be removed from a polycondensation system by means of a simplified dehydration operation and apparatus.
2. Description of the Related Art
Polyarylene sulfides, such as polyphenylene sulfides having a recurring unit as represented by formula: ##STR1## are mostly thermoplastic resins, which have partial thermosetting chracteristic. They have excellent properties as engineering plastics, such as high chemical resistance, mechanical properties over a wide range of temperatures, and thermal rigidity.
Such polyarylene sulfides can generally be prepared by polycondensing a dihalogenated aromatic compound with a sulfur source (a sulfur compound) in an organic polar solvent. As the sulfur source, there may be used mainly an alkali metal sulfide or hydrosulfide, particularly sodium sulfide or sodium hydrosulfide. The alkali metal sulfides and hydrosulfides are industrially available in the form of a hydrate or an aqueous mixture in many cases. Hence, the use of such industrially available alkali metal sulfides provides the polycondensation system for synthesizing the polyarylene sulfides with water, too, thereby resulting in a decrease in molecular weight of the resulting products and an increase in a content of impurities within the resulting products. Such polyarylene sulfides are poor in thermal resistance, resistance to ultraviolet rays and so on, so that they cannot be formed into films, sheets, fibers and moldings with high physical properties. Therefore, dehydration operation has conventionally been performed, for example, by using a stream of nitrogen, prior to polycondensation in order to avoid such drawbacks.
The dehydration operation in a nitrogen stream, however, suffers from the disadvantages that, when it is performed at low temperatures, on the one hand, a long period of time is required for dehydration so that it is not available on an industrial basis and, when it is carried out at high temperatures, on the other hand, the raw materials may be caused to be decomposed. Thus, the dehydration operation has been carried out in a nitrogen stream at mild temperature conditions, however, this dehydration results in the presence of a water residue in the amount ranging from 1 mole to 1.5 moles per mole of the sulfur source used within the system.
A process for controlling the amount of water residue to less than 1 mole prior to polycondensation is proposed, for example, by Japanese Patent Unexamined Publication (kokai) No. 98,133/1984, which is directed to a process for preparing polyphenylene sulfides having a recurring unit as represented by formula: ##STR2## involving reacting a p-dihalo- benzene with a source of sulfur in a polar solvent of an organic amide type in such a state that the water content in the system is controlled to 0.3-0.95 mole per mole of sulfur. This process, however, requires dehydration of the system to be carried out prior to addition of the p-dihalobenzene to the polycondensation system, and the dehydration operation is performed by distillation so as to reduce the amount of the water content within the system prior to polycondensation to 0.3-0.95 mole per mole of sulfur. This publication states the reasons for adjusting the water content in the system within the predetermined scope prior to polycondensation, because if the water content remaining within the system would be lower than 0.3 mole per mole of sulfur, on the one hand, the degree of polycondensation of the resulting polyphenylene sulfide is unlikely to be raised, thereby leading to the preparation of polyphenylene sulfides which are not very suitable for molding and so on. The polyphenylene sulfides with such a low degree of polycondensation requires its apparent melt viscosity to be raised by means of cross-linking or the like, however, such polyphenylene sulfides as having an elevated apparent melt viscosity is likely to deteriorate. This publication also states that, if polycondensation would be carried out in such a system that the water content remains in the system Prior to polycondensation in the amount as high as more than 0.95 mole per mole of sulfur, on the other hand, polyphenylene sulfides result which contain an increased amount of oligomers so that oxidative cross-linkability, decomposition and cross-linkability upon exposure to ultraviolet rays, and deterioration by heat may be increased. It further states that distillation may preferably be carried out by means of a fractionating column or tower. As disclosed in the above publication, the process has the problems that a special apparatus such as the fractionating column or tower is required and that it takes a long period time for dehydration.
Further, our copending Japanese Patent Applications No. 195,844/1988 (Unexamined Publication (kokai) No. 45,531/1990) discloses improvements in processes for dehydration, which involve controlling the water content in the system prior to polycondensation to less than 1 mole per mole of a sulfur source by means of a dehydration operation under reduced pressures or by using a dehydrating column or the like. These improved processes, however, still have room for an improvement in terms of application of reduced pressures to the dehydration process and the use of a special device such as a dehydrating column or the like.
A further aspect of the process for preparing polyarylene sulfides is disclosed in Japanese Patent Unexamined Publication (kokai) No. 212,429/1985, which involves carrying out an optionally continuous reaction in a cascade reactor system consisting of two to six reactors under optionally elevated pressures so as to remove water in the mixture prior to reaction or in a preparatory step for the reaction in the presence of all reaction components and then continuously elevating the reaction temperature for each of the individual reactors. As will be apparent from the foregoing description, too, this process suffer from the disadvantage that a plurality of reactors are required, thereby resulting in a, complexity of the construction of the system as a whole, for which control is so complicated as to continuously elevate the temperatures for the individual reactors.
A still further aspect of the process for preparing polyarylene sulfides is disclosed in Japanese Patent Unexamined Publication (kokai) No. 105,027/1984, which involves carrying out the reaction step in a batchwise manner constantly under slightly higher pressure in a mole ratio of an alkali metal sulfide to an organic solvent of 1:2 to 1:15 and removing water from the reaction mixture azeotropically. This publication further states that the reaction be batchwise carried out by heating the reaction mixture at 160.degree. to 270.degree. C. Actually, when a solvent having a relatively low boiling point, such as N-methylpyrrolidone, is used as an organic solvent, however, a limit should be placed upon this process in terms of the temperature for polycondensation because this reaction should be carried out under substantially ambient pressure, although under slightly elevated pressure. This process further has the technical difficulty that it requires a long time period for polycondensation. Furthermore as is apparent from the working examples disclosed in this publication, this process require dehydration prior to polycondensation at temperatures as substantially high as boiling points of the solvents for polycondensation used and thereafter polycondensation for a long period of time. Hence, this process eventually is not very different from the process as disclosed in Japanese Patent Unexamined Publication No. 212,429/1985 which requires a plurality of the reactors as described hereinabove.
Briefly speaking, such conventional processes as described hereinabove are all based on the basic concept that water contained in the source of sulfur be removed prior to polycondensation.